" Record numbers of electors are applying for postal votes for the general election, raising fears that it will be open to widespread fraud.
A survey by the Guardian of 55 councils covering 135 constituencies reveals applications to vote by post have risen in all cases, tripling in some places, particularly in inner cities.
The increase comes as demand grows for urgent changes in the postal-voting system, last week labelled by a judge as "an open invitation to fraud". There is an ongoing court case, and police are investigating fraud in six areas of the country.
Numbers of applications for postal votes are likely to increase further once the date of the election is announced - widely expected to be May 5.
Doubts over the safety of postal voting have been highlighted by several recent cases. Judge Richard Mawrey QC, presiding over hearings into allegations of widespread, organised postal voting fraud in the local elections in Birmingham last June, said last week the system was "an open invitation to fraud".
He has heard evidence of wholesale theft of votes in the city, with thousands of postal ballots being diverted to a "safe house", where they were allegedly filled in on an "industrial scale". He concluded: "Someone who was so inclined could defraud the system."
The Guardian survey reveals that in areas where allegations of postal-voting fraud have been made in the past - the "hot spots", according to one local government official - increases in applications to vote by post are among the highest.
In Birmingham's 11 constituencies, more than 53,000 people have asked to vote by post, compared with 16,000 at the last election.
In Woking, where police have begun an investigation into allegations of postal-voting fraud at June's local elections, 15,000 electors are asking to vote by post, compared with 2,356 in the last election.
The Electoral Commission, which has called for urgent changes to the system to increase security, is hoping guidelines from the Association of Chief Police Officers will help detect and prevent abuses before the general election.
Police forces in marginal constituencies such as Basingstoke (Conservative majority 880), Cheadle (Liberal Democrat majority 33), Dorset South (Labour majority 153), and Perth (Scottish Nationalist Party majority 43), are likely to be put on the highest alert.
But returning officers say they need the powers and resources to investigate.
Malcolm Dumper, executive director of the Association of Electoral Administrators and the returning officer in Southampton, where applications for postal votes have risen from 2,000 at the last general election to 26,000, said: "I organise the election in constituencies with high student populations. Many of them live in houses of multiple occupancies, so you can have 20 postal ballot papers being sent to one address. But we don't have the facilities to check these addresses and voters, we don't have the powers or the machinery to check the validity of the applications."
The government has defended the postal-voting system and changed the law in 2000 to make it available on demand. But recent cases have shown how easy it is for large numbers of ballot papers to be sent to one address where they are filled out fraudulently or for party supporters to collect papers from householders and fill them in for them.
Inner-city areas with high ethnic minority populations are particularly vulnerable to manipulation. Ayoub Khan, prospective parliamentary candidate for the Liberal Democrats in Birmingham Ladywood, said: "An aspect of the culture is the hierarchical system, which extends to grandparents and extended family in Pakistan. Key members of the community, who either have party affiliations or have relatives who do, will tell the family they want all these postal votes to be given to a certain individual."
The police are investigating allegations of postal voting fraud in Cheshire, Derbyshire, Greater Manchester, West Yorkshire and Woking. In Reading, police have concluded an investigation which found there was fraud but said there was no evidence to bring charges against individuals.
A former Labour councillor in Blackburn faces a possible prison sentence after admitting to conspiracy to rig the 2002 town hall election by getting supporters to fraudulently fill in more than 200 blank postal votes.
"The problem is that the postal-voting system is simply not secure enough," said Lord Greaves, a Liberal Democrat peer who has called for international observers to oversee the general election."